The present invention relates to a process for the simultaneous production of a liquefiable natural gas and a natural gas liquids (NGL) cut from a feed natural gas containing nitrogen, methane, C2 to C5 hydrocarbons and C6+ heavy hydrocarbons, of the type comprising the following steps:                (a) the said feed natural gas is pretreated in order to obtain a pretreated natural gas;        (b) the pretreated natural gas resulting from step (a) is cooled down to a temperature close to its dew point;        (c) the cooled pretreated natural gas resulting from step (b) is expanded and the expanded natural gas is introduced into an NGL recovery unit comprising at least one main distillation column so as to produce, on the one hand, as column top product, a purified natural gas and, on the other hand, the said NGL cut; and        (d) the said liquefiable natural gas is formed from the purified natural gas resulting from step (c).        
The process of the present invention applies to plants for producing, from a natural gas extracted from under the ground, liquid natural gas (termed an “LNG”) as main product and a cut of natural gas liquids (that will be termed “NGL”) as by-product.
In the present invention, NGL is understood to mean C2+ to C3+ hydrocarbons that can be extracted from natural gas. By way of example, these NGLs may comprise ethane, propane, butane and C5+ hydrocarbons.
The LNG produced after extraction of the NGLs possesses a lower calorific value than an LNG produced without extraction of the NGLs.
Known natural gas liquefaction plants comprise, in succession, a unit for producing a liquefiable gas, a unit for the actual liquefaction, and a unit for removing nitrogen from the LNG. The unit for producing a liquefiable gas necessarily comprises means for removing the C6+ heavy hydrocarbons that may crystallize during liquefaction.
To produce liquefiable natural gas and NGLs simultaneously, it is possible to use, for example, a process of the aforementioned type, such as that described in Application FR-A-2 817 766.
Such a process has a thermodynamic efficiency optimized for producing a natural gas at room temperature and for NGL extraction.
Consequently, this process is not entirely satisfactory if the natural gas obtained has to be liquefied. This is because the energy expenditure needed to liquefy the natural gas obtained is relatively high.